How to update source tree at /usr/src using svn on FreeBSD

Originally published at: https://www.cyberciti.biz/faq/update-source-tree-at-usr-src-using-svn-on-freebsd/

I compiled custom FreeBSD kernel for my FreeBSD jail to use vnet. How do I update my FreeBSD /usr/src/ code tree? How can I update source tree at /usr/src on FreeBSD using svn command to patch and compile kernel again?